The present invention provides a relatively simple, easy and accurate method of measuring optical densities up to values of 10 or 11. This has previously not been possible. With existing densitometers, a density of 5 or 6 was the highest that could be measured, and the accuracy at high values was questionable. This invention has a potential for wide application in industries requiring accurate optical density values of materials with a high optical density and is equally applicable in all density (or transmission) measurements.
The present laser densitometer system is a substantial advancement in the state-of-the-art of spectrophotometric measurements of highly optically dense samples. Novel use is made of a single pulsed light source (such as a Q-switched laser or a pulse laser pumped dye line), and a single detector system. The invention permits the optical density measurement of very opaque samples having transmission values as low as 10.sup.-11. It can be readily automated and applied to scanning problems. A cost reduction over previous techniques is possible.
It is emphasized that laser sources are finding increasing application in spectroscopy because of their higher spectral brightness and purity. This invention is a spectrophotometric application where the laser's special temporal characteristics are also important, i.e., a simple and sensitive laser optical densitometer. The new densitometer uses a pulsed dye laser and an optical delay line.
The availability of intense laser light sources with a broad wavelength range, such as obtained from nitrogen laser pumped dye cavities, permits transmission spectrophotometry through very optically dense samples. Previously, conventional measurements required very sensitive differential techniques and stable sources. The particular advantages of a pulse nitrogen laser-pumped dye cavity in densitometry (e.g., Molectron UV-100 N.sub.2 laser and Molectron Model DL-400 dye cavity) are the: high peak power (.about. 50 kw); high pulse rate (50 Hz), and short pulse duration (5 to 10 ns). Possibly mode-locked cavities could also be applied as described subsequently; however, special instrumentation for adequate measurements of the ultra-short pulses would be required.
The nominal pulse duration of a pulsed dye laser is just sufficiently short to permit the construction of an optical delay of typical laboratory dimensions. In this specific example, an air path was used but optical fiber bundles can serve just as well. The delayed path is used for a reference signal in a transmission measurement, thus eliminating the need for an additional detector and subsequent precision control of the detector amplifier gain. A single laser pulse is used for both the probe and the reference. The laser stability, therefore, can be conveniently ignored.
Applications are immediate in the testing of eyewear for the protection against laser light. The present system permits a very accurate assay of the transmission properties of various optical filters and attenuators, and due to its high precision permits the monitoring of very small changes on a very dense sample as a function of various stresses such as chemical, heat, light, and aging. It allows the measurement of transmission of infrared (IR) or ultraviolet (UV) light pulses through aircraft windscreens. In the biomedical field, it will permit safe and convenient optical probing of the human body which can be modeled as a varying optical density filter and permit imaging by applying scanning techniques and thereby substitute x-ray methods.